In a four wheel vehicle, the front wheels are steered for changing the traveling direction of the vehicle, and are additionally given with a certain toe angle so as to ensure a motion stability of the vehicle. In recent years, it has also been proposed to control the toe angle of the rear wheels for the purpose of improving the stability of the vehicle when braking and the responsiveness when accelerating, and improving the heading response of the vehicle at the time of a low speed turn and the stability of the vehicle at the time of a high speed turn.
A typical toe angle control device mounted on such a vehicle includes an electric linear actuator interposed between the base end of a lateral link or trailing link of each rear wheel and the opposing part of the vehicle body so that the effective length of the lateral link or trailing link may be varied. See Japanese patent laid open publication No. 9-30438 (patent document 1).
A road wheel is typically fitted with a pneumatic tire made of synthetic rubber and filled with compressed air so that the ride quality of the vehicle may be ensured even when traveling over irregular road surfaces and the rolling resistance of the wheel may be controlled by minimizing the contact area with the road surface. As the condition of a tire strongly affects the driving performance of the vehicle, it is desirable to monitor the condition of the tire. Various proposals have been made. One such proposal is based on the use of a detecting the rotational speed of the wheel by forming a cogged wheel concentric to the wheel and counting the teeth of the cogged wheel with a suitable sensor attached to a stationary part of the vehicle adjacent to the cogged wheel, and comparing the detected rotational speed with a reference value. See Japanese patent laid open publication No. 10-71819 (patent document 2).
It is known that the pneumatic pressure of a tire can decrease over time depending on the mileage, driving condition and road condition, and this may impair the performance of the tire. When a tire becomes unusable due to air leakage or puncture, a temporary spare tire may be used. A temporary tire is typically significantly lower in performance than a normal tire. Also, a temporary tire typically wears much faster than a normal tire, and this further diminishes the performance of the tire. When the performance of one of the tires of a vehicle drops, the cornering power of the affected tire becomes lower than that of the unaffected laterally opposing tire. This is manifested when making a turn. The steering property differs depending on the direction of the turn, and a vehicle operator experiences a significant difficult in slaloming the vehicle in a stable manner.